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United States Patent |
6,058,338
|
Agashe
,   et al.
|
May 2, 2000
|
Method and apparatus for efficient GPS assistance in a communication
system
Abstract
A method and apparatus for efficiently transmitting location assistance
information to a mobile communication device over a control channel with a
minimal impact on the capacity of the control channel. A position location
server provides a difference between satellite locations which have been
computed using Almanac data and then Ephemeris data. Sending only the
difference between the locations and clock corrections computed using the
two different data types, the total amount of information to be
transmitted to a mobile communication device is significantly reduced.
Furthermore, by providing rate of change information, the method and
apparatus allows the location assistance information to remain valid for a
relatively long time after it is has been received by the mobile
communication device.
Inventors:
|
Agashe; Parag A. (San Diego, CA);
Vayanos; Alkinoos Hector (San Diego, CA);
Soliman; Samir S. (San Diego, CA)
|
Assignee:
|
Qualcomm Incorporated (San Diego, CA)
|
Appl. No.:
|
250771 |
Filed:
|
February 12, 1999 |
Current U.S. Class: |
701/13; 244/158R; 342/355; 356/139.01; 701/213 |
Intern'l Class: |
G06F 007/70 |
Field of Search: |
701/13,213
244/158 R
356/139.01
342/355
|
References Cited
U.S. Patent Documents
5430657 | Jul., 1995 | Kyrtsos | 364/459.
|
5862495 | Jan., 1999 | Small et al. | 701/13.
|
Primary Examiner: Cuchlinski, Jr.; William A.
Assistant Examiner: Hernandez; Olga
Attorney, Agent or Firm: Miller; R. Ben, Greenhaus; Bruce W.
Claims
What is claimed is:
1. A method for calculating the position of a satellite, including the
steps of:
a) transmitting Almanac data to a mobile communication device;
b) computing the location of the satellite at a first point in time using
the transmitted Almanac data;
c) computing the clock correction for the satellite at the first point in
time using the transmitted Almanac data;
d) computing the location of the satellite at the using Ephemeris data at
the first point in time;
e) computing the satellite clock correction for the satellite at the first
point in time using the Ephemeris data;
f) computing difference between the location and satellite clock correction
computed using the transmitted Almanac data and the location and clock
correction using the Ephemeris data; and
g) transmitting to the mobile communication device the computed
differences.
2. A server for calculating information that assists in locating the
position of satellites, including:
a) an output port capable of outputting an Almanac and satellite location
information, including information regarding the difference between clock
corrections and locations computed based upon Almanac and Ephemeris data;
and
b) a processor, coupled to an output port, capable of:
i) computing the location of a satellite based upon the Almanac;
ii) computing the satellite clock correction based upon the Almanac;
iii) computing the location of the satellite based upon the Ephemeris;
iv) computing the satellite clock correction based upon the Ephemeris; and
v) computing the difference between the location and satellite clock
corrections which were computed based upon the Almanac and the Ephemeris.
Description
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to position location systems. More
particularly, the present invention relates to systems and methods for
determining the physical location of a mobile telephone within a cellular
communication system.
II. Description of the Related Art
Recent developments in global positioning satellite (GPS) systems and
terrestrial mobile communications make it desirable to integrate GPS
functionality into a mobile communication device, such as a mobile
telephone, in order to support various position location functions and
features. A wireless link exists between a mobile communication device
within a terrestrial mobile communications system and a base station
within the communications system. A base station is typically a stationary
communication device that receives wireless communications from, and
transmits wireless communications to, a wireless mobile communication
device. The base station also communicates with communication networks to
complete the connection between the mobile communication device and an
end-point device, such as another mobile communication device, a
conventional telephone, a computer or any other such device. This wireless
link may be used to communicate position location information between the
mobile communication device and the base station in order to improve the
performance of the GPS receiver within the mobile communication device. In
particular, certain functions that must be performed in order to locate
the position of a mobile communication device in accordance with a GPS
system can be performed by the base station, rather than by the
communication device. By "off-loading" some of the functions to the base
station, the complexity of the communication device can be reduced.
Furthermore, since the base station is stationary, the location of the
base station can be used to assist in locating the position of the
communication device.
Many services, such as CDMA Tiered Services (described in industry standard
TR45.5.2.3/98.10.xx.xx, CDMA Tiered Services Stage 2 Description, Version
1.1, published by the Telecommunication Industry Association/Electronics
Industry Association (TIA/EIA)), require a wireless telephone to be
capable of determining its location while in an idle state. The location
must then be displayed to the user. In the idle state, the wireless
telephone monitors transmissions from a base station over a control
channel broadcast by the base station. For example, in an industry
standard IS-95 CDMA system (as defined by industry standard IS-95,
published by the TIA/EIA), the base station transmits a paging channel.
Each of the telephones capable of receiving signals from a particular base
station will monitor information broadcast on the control channel to
determine whether incoming calls or other data are intended for that
telephone.
A GPS receiver typically measures the range to at least four GPS
satellites. If the locations of the satellites and the ranges from the
phone to the satellite are known at the time the measurement is made, then
the location of the phone can be computed. Since GPS satellites orbit
around the Earth, the relative position of the GPS satellites with respect
to the earth changes with time. The location of a GPS satellite can be
determined by having a description of the orbit of the satellite along
with the time when the satellite position is to be computed. The orbits of
GPS satellites are typically modeled as a modified elliptical orbit with
correction terms to account for various perturbations.
In a GPS system, the orbit of the satellite can be represented using either
an "Almanac" or an "Ephemeris". An Ephemeris provides data that represents
a very accurate representation of the orbit of the satellite. An Almanac
provides data that represents a truncated reduced precision set of the
parameters provided by the Ephemeris. Almanac data is much less accurate
than the detailed Ephemeris data. Almanac accuracy is a function of the
amount of time that has elapsed since the transmission. Table 1 shows the
relationship between the age of the information (i.e., amount of time
which has elapsed since the information was transmitted) and the accuracy
of the information.
TABLE 1
______________________________________
Age of data time
(from transmission)
Almanac Accuracy (m)
______________________________________
1 day 900
1 week 1200
2 weeks 3600
______________________________________
In addition, the Almanac provides truncated clock correction parameters.
The almanac time correction provides the time to within 2 .mu.sec of GPS
time. However, the satellite location and clock correction computed using
Almanac data are not useful to compute the location of the phone because
of the low accuracy as shown in the above Table 1.
Certain methods for computing the location of a device require measuring
the ranges to the satellites at the wireless phone, and then transmitting
these ranges to a server connected to the base station. The base station
uses these ranges, along with the locations of the satellites at the time
the range measurements were made, to compute the location of the phone.
This computed location may be displayed to the user or sent to any other
entity that needs the location. This method is suitable for a phone that
has a dedicated traffic channel. However, this method is not suitable for
phones in the idle state, because the phone lacks a dedicated traffic
channel over which to send the information to the base station during idle
state.
In the absence of a dedicated traffic channel over which to communicate
with the base station, the phone may use a shared access channel to send
information to the base station. However, transmitting measured ranges to
the base station over the shared access channel (which is commonly used to
establish a call to or from the phone) can have a significant impact on
the capacity of the shared access channel and on the life of the battery
that powers the phone. Hence, it is not practical to transmit measured
ranges to the base station. This requires that the phone be able to
compute its own location. In order to do so, the phone must know the
locations of the GPS satellites, and the errors in the GPS satellite clock
(since an accurate GPS satellite clock is required to determine the range
measurements accurately). This information must be transmitted to the
phone over the control channel. However, even transmitting this
information to the phone creates a significant burden on the control
channel.
Under conventional conditions, the control channel has to carry very large
amounts of information. The control channel has a limited capacity to
carry messages. Hence, it is not possible to convey extensive GPS
information over the control channel. Furthermore, the information must be
transmitted in a form that allows the information to be used for a
relatively long time after it has been received.
These problems and deficiencies are recognized and solved by the present
invention in the manner described below.
SUMMARY OF THE INVENTION
This method and apparatus disclosed herein efficiently transmits location
assistance information to a mobile communication device over a control
channel with a minimal impact on the capacity of the control channel. A
position location server provides a difference between satellite locations
which have been computed using Almanac data and then Ephemeris data.
Sending only the difference between the locations and clock corrections
computed using the two different data types, the total amount of
information to be transmitted to a mobile communication device is
significantly reduced. Furthermore, by providing rate of change
information, the method and apparatus allows the location assistance
information to remain valid for a relatively long time after it is has
been received by the mobile communication device.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects and advantages of the present invention will become
more apparent from the detailed description set forth below when taken in
conjunction with the drawings in which like reference characters identify
correspondingly throughout and wherein:
FIG. 1 is high level block diagram of the components of a communication
system using a satellite position location system (such as a GPS system)
to locate a mobile communication device.
FIG. 2 is a high level block diagram of the mobile communication device in
accordance with the disclosed method and apparatus.
FIGS. 3a-3c illustrate the steps to be performed by the disclosed method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This disclosed method and apparatus provides a novel way to reduce the
amount of information that needs to be transmitted between a mobile
communication device and a base station. In particular, the disclosed
method and apparatus reduces the amount of information required to be
transmitted by using two types of information available for determining
position location. The first type of information is less accurate, but
more efficient information (such as Almanac data provided by global
positioning satellites (GPS)). The second type of information is more
accurate, but less efficient information (such as Ephemeris data provided
by GPS satellites). The disclosed method and apparatus minimizes the
impact on the capacity of a communication channel (such as a control
channel), and, in accordance with one embodiment, maximizes the amount of
time for which the transmitted information is valid.
FIG. 1 is high level block diagram of the components of a communication
system using a satellite position location system (such as a GPS system)
to locate a mobile communication device. The communication system includes
a mobile communication device 100 and a base station 102. The mobile
communication device 100 may be any device that is capable of
communicating with a base station over a wireless connection (such as a
wireless telephone, computer with wireless modem, or facsimile machine
with wireless modem). The base station 102 is any device that is capable
of receiving wireless transmissions from the mobile communication device
100. Typically, such a base station 102 will interface the mobile
communication device 100 with a land based communication network, such as
the public switched telephone network (PSTN) or the internet.
In accordance with one embodiment of the disclosed method and apparatus,
the base station 102 includes a position location server 106 (such as a
GPS server). Alternatively, the position location server 106 is located
apart from the base station 102 and communicates with the base station 102
over a communication link. The communication link between the base station
102 and the server 106 may take any form that allows information to be
communicated from the mobile communication device 100 to the server 106
via the base station 102. In one alternative embodiment of the disclosed
method and apparatus, the functions described herein as being performed by
the position location server 106 are performed directly by the base
station 102. Accordingly, in such an embodiment, no discrete position
location server is required.
FIG. 2 is a high level block diagram of the mobile communication device
100. The mobile communication device 100 includes a position location
antenna 300, a position location receiver 302, a position location
processor 304, a communication antenna 306, a communication receiver 308,
a communication processor 310, and a memory 312.
FIG. 3a through FIG. 3c illustrate the steps performed in accordance with
the disclosed method. Initially, the position location server 106
transmits to the mobile communication device 100, information regarding
the orbits of each of the satellites 104 from which the mobile
communication device 100 is likely to receive position location signals
(STEP 300). In accordance with one embodiment, this information is
communicated by the position location server 106 to the base station 102.
The information is then transmitted over the air from the base station 102
and received by the communication antenna 306 within the mobile
communication device 100. The signals received by the communication
antenna 306 are coupled to the communication receiver 308. The
communication receiver 308 performs any necessary radio frequency
processing (such as filtering, down converting, amplification, etc.). Such
radio frequency processing is well known to those skilled in the art. The
output from the communication receiver 308 is coupled to the communication
processor 310.
The communication processor 310 extracts from the received signals the
information regarding the orbits of the position location satellites.
Extraction of this information is well known to those of ordinary skill in
the art. This information is provided in the well known "Almanac" format.
Together with the Almanac, the mobile communication device 100 receives an
"Almanac Identifier". The Almanac Identifier uniquely identifies the
Almanac with which the Identifier was transmitted. The mobile
communication device 100 stores the Almanac and an Almanac Identifier in
its memory 312 (STEP 302). In accordance with one embodiment of the
disclosed method and apparatus, the Almanac Identifier is a number that
represents the period of time (such as a week) for which the Almanac is
valid.
At any time relative to transmitting the Almanac and Almanac Identifier to
the mobile communication device 100 (either before, during or after such
transmission), the position location server 106 uses the transmitted
Almanac to compute the location of a satellite to which the Almanac is
relevant (STEP 304). In addition, a clock correction is calculated using
the Almanac. The clock correction is calculated at a time t.sub.0. Such
clock correction accounts for clock errors that result from errors in the
satellite clock. Methods for calculating such clock corrections are well
known to those skilled in the art.
The location of a particular satellite 104 at time t.sub.0 as computed from
the Almanac is denoted as (x.sub.0a, y.sub.0a, z.sub.0a). The clock
correction for the satellite computed from the Almanac at time t.sub.0 is
denoted as C.sub.0a.
Next, the position location server 106 computes the location of the
satellite and the clock correction using well known "Ephemeris" data at
time t.sub.0 (STEP 306). It will be understood by those skilled in the art
that Ephemeris data is necessary in any accurate GPS position location
system. Both Ephemeris data and Almanac data are received from the
Satellite in real-time. The location of the satellite 104 computed from
the Ephemeris at time t.sub.0 is denoted as (x.sub.0e, y.sub.0e,
z.sub.0e). It should be understood that the satellite for which the values
(x.sub.0a, y.sub.0a, z.sub.0a) were computed (i.e., the satellite from
which the Almanac was received) is the same as the satellite for which the
values (x.sub.0e, y.sub.0e, z.sub.0e) were computed (i.e., the satellite
from which the Ephemeris was received ). The clock correction for the
satellite computed from the Ephemeris at time t.sub.0 is denoted as
c.sub.0e.
The position location server 106 also computes the differential correction
to be applied to the range measured from the satellite 104 (STEP 308). The
differential correction is applied to correct for clock errors that are
intentionally introduced by the satellite for historical reasons which are
unrelated to the disclosed method and apparatus. The differential
correction is denoted as d.sub.0.
The position location server 106 computes the difference between the
location x, y and z and the clock correction of the satellite obtained
from the Almanac and the Ephemeris (STEP 310). The difference in location
at time t.sub.0 is expressed as:
.DELTA.x.sub.0 =x.sub.0e -x.sub.0a
.DELTA.y.sub.0 =y.sub.0e -y.sub.0a
.DELTA.z.sub.0 =z.sub.0e -z.sub.0a
.DELTA.c.sub.0 =c.sub.0e -c.sub.0a
The base position location server 106 also computes a value,
.DELTA.c.sub.0d =.DELTA.c.sub.0 +d.sub.0 which represents the clock
correction .DELTA.c.sub.0 after differential correction has been added
(STEP 312). The corrected clock value, .DELTA.C.sub.0d is then used to
adjust the values of .DELTA.x.sub.0, .DELTA.y.sub.0, and .DELTA.z.sub.0
(STEP 314).
The base station computes the rate of change of the corrections,
.DELTA.x.sub.0, .DELTA.y.sub.0, .DELTA.z.sub.0, and .DELTA.c.sub.0d (STEP
316). We denote the rate of change of .DELTA.x.sub.0 as .DELTA..sub.0, the
rate of change of .DELTA.y.sub.0 as .DELTA..sub.0, the rate of change of
.DELTA.z.sub.0 as .DELTA..sub.0, and the rate of change of .DELTA.c.sub.0d
as .DELTA..sub.0d. In accordance with one embodiment of the disclosed
method and apparatus, the rate of change values are computed by taking the
difference between two locations at two point in time and identifying the
slope of the line between them in each direction x, y, and z.
The position location server 106 sends t.sub.0, .DELTA.x.sub.0,
.DELTA.y.sub.0, .DELTA.z.sub.0, .DELTA.c.sub.0d, .DELTA..sub.0,
.DELTA..sub.0, .DELTA., .DELTA..sub.0d for each satellite to the mobile
communication device (STEP 318). In accordance with one embodiment of the
disclosed method and apparatus, these values are sent via a control
channel, such as the shared access channel defined by industry standard
IS-95B, published by the Telecommunications Industry
Association/Electronics Industry Association (TIA/EIA). Along with this
information, the position location server 106 sends an Identifier to
identify the Almanac that the server 106 used in computing these
corrections.
The mobile communication device receives the information over the control
channel. The mobile communication device compares the Identifier of the
Almanac used by the server 106 with the Identifier of the Almanac stored
in its memory 312 (STEP 320).
If the Identifiers match (STEP 322), then the mobile communication device
computes the location of each satellite 104 and each satellite clock
correction using the Almanac at time t.sub.1 (STEP 324). Time t.sub.1 may
not necessarily be the same as time t.sub.0. The satellite locations and
clock correction computed by the phone at time t.sub.1 are denoted as
(x.sub.1a, y.sub.1a, z.sub.1a), and c.sub.1a respectively.
The mobile communication device applies the corrections received over the
control channel to the satellite locations and satellite clock computed
using the Almanac (STEP 326). This yields the corrected satellite location
and clock correction. These can be written as:
##EQU1##
The mobile communication device uses these corrected satellite locations
and clock along with range measurements to the satellite to compute its
own location. The position location signals are received by the position
location antenna 300. The position location antenna 300 is coupled to the
position location receiver 302. The position location receiver 302
performs any necessary front end radio frequency processing. The position
location receiver 302 is coupled to the position location processor 304.
The position location processor determines the distance to each satellite
104 in conventional fashion and then determines its own position using the
locations of each satellite 104.
If the Identifier of the Almanac used by the base station does not match
(STEP 322) the Identifier of the almanac stored in the mobile
communication device's memory 312, the mobile communication device sets up
a traffic channel (STEP 328), and downloads the new Almanac from the
server 106 via the base station 102 (STEP 330).
In the preferred embodiment of the disclosed method and apparatus, the
information sent over the control channel includes the location and clock
corrections, and their first order differentials with respect to time
(rate of change) .DELTA..sub.0, .DELTA..sub.0, .DELTA..sub.0 and
.DELTA..sub.0d. An alternate embodiment may include higher order
differentials with respect to time. Another alternate embodiment may
exclude the rate of change terms altogether.
In the preferred embodiment, the differential correction for the ranges
measured by the phone is accomplished by applying a correction to the
clock correction term .DELTA.c.sub.0 to obtain .DELTA..sub.0d. In other
embodiments, differential correction of the ranges can be achieved by
applying correction terms to the satellite location corrections
.DELTA.x.sub.0, .DELTA.y.sub.0, .DELTA.z.sub.0 instead of to the clock
correction term .DELTA.c.sub.0.
Although the invention refers specifically to a Global Positioning System,
these same principles can be applied to other satellite based location
systems such as GLONASS.
Furthermore, references to IS-95 CDMA systems are provided only as an
example of a particular communication system. However, the disclosed
method and apparatus has applicability to other wireless communications
systems, where it is desirable to reduce the amount of information to be
transmitted between an mobile communication device and a position location
server.
Since the only information transmitted over the control channel is the
difference between the location and clock correction computed from the
Ephemeris and the Almanac a small number of bits can be used to convey
this information. Other methods of sending the satellite location
information such as sending the Ephemeris parameters or the actual
location of the satellite require many more bits to transmit the
information. Such methods are much more expensive in terms of capacity of
the control channel.
The corrections transmitted to the mobile communication device in disclose
method and apparatus are valid for a long window of time after the time of
transmission due to the inclusion of rate of change information. Hence,
the mobile communication device can use this information at a time
different from the time when it was transmitted. This means that the
mobile communication device can compute its own location at any time, and
is not restricted to location computation within a small window of time
around the time when the information is transmitted. Other ways of sending
the satellite information are valid only for a very short window of time
after they are transmitted.
The disclosed method and apparatus is provided to enable any person skilled
in the art to make or use the present invention. The various modifications
to the disclosed method and apparatus will be readily apparent to those
skilled in the art, and the generic principles defined herein may be
applied to other embodiments without the use of inventive faculty. Thus,
the present invention is not intended to be limited to the methods and
apparatuses shown herein but is to be accorded the widest scope consistent
with the claims set forth below.
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